1. Field of the Invention
The present invention relates to a glass composition for a substrate, which is useful as a substrate glass for flat display panels, particularly for plasma display panels (PDP) and which is suitable for production of a large size substrate by a float process.
2. Discussion of Background
PDP is usually prepared by baking metal electrodes, an insulating paste, a rib paste, etc. on a substrate glass at a maximum temperature of from about 550.degree. to 600.degree. C., followed by frit-sealing a facing plate. As a substrate glass for this purpose, it has heretofore been common to employ soda lime silica glass which is widely used in the field of buildings or automobiles.
However, the glass transition temperature of such soda lime silica glass is at a level of from 530.degree. to 550.degree. C. Therefore, if subjected to heat treatment at the above maximum temperature, the substrate undergoes deformation, or the substrate glass undergoes a substantial dimensional change, whereby it has been difficult to precisely align the electrode positions with the facing plate. Such a problem tends to be more distinct when the size of the panel becomes large to a level of e.g. a so-called 40 inch panel, and a substrate glass having higher heat resistance will be required.
Alkali-free glass for liquid crystal, as disclosed in e.g. U.S. Pat. No. 5,348,916, is available as a substrate glass which is presently widely used as glass for display and which is superior in the heat resistance to the soda lime silica glass. However, the thermal expansion coefficient of such glass is within a range of from 35.times.10.sup.-7 to 50.times.10.sup.-7 .degree. C..sup.-1 which is far lower than the thermal expansion coefficient of from 80.times.10.sup.-7 to 90.times.10.sup.-7 .degree. C..sup.-1 of the soda lime silica glass. Therefore, in order to use such alkali-free glass for liquid crystal, for PDP, it will be necessary to properly select various frit-paste materials so that they agree to the thermal expansion coefficient of the substrate glass. However, it is practically very difficult to select them within the above temperature range.
On the other hand, Japanese Unexamined Patent Publication No. 40933/1991 discloses a glass composition which has a thermal expansion coefficient of substantially the same level as the thermal expansion coefficient of the soda lime silica glass and which has a small thermal deformation at 600.degree. C. However, the composition disclosed in this publication is still not fully satisfactory for either one of the following reasons.
(1) The glass transition temperature is lower than 600.degree. C., the shrinkage of the glass by heat treatment is not sufficiently small for a panel as large as a so-called 40 inch panel.
(2) The content of CaO is at least 6 wt %, whereby the devitrification temperature is high, and it is difficult to form a panel excellent in the quality and productivity by a float process.